In conventional artillery it is known to employ ammunition with metal cartridge cases, made of brass in most cases.
While the metal cartridge case has undoubted advantages where the robustness and the leaktightness at the rear of the shell at the time of firing are concerned, it nevertheless has many disadvantages.
Being made of metal, it does not take part in the combustion of the propellent charge and represents a useless dead space from the viewpoint of ballistics. Furthermore, after the propellent charge has burnt, it contains many toxic gases, and this represents a certain handicap when it has to be extracted and kept inside a closed enclosure such as, for example, a battle tank. Finally, since the metal of which it consists is generally a copper-based alloy, its manufacturing cost is high.
To get rid of these disadvantages, one solution consists in employing entirely combustible cartridge cases. The combustible cartridge case, in fact, offers the twin advantage of taking part in the combustion of the propellent charge and, in doing so, of contributing additional energy and of leaving no solid space containing toxic gases after combustion. For these reasons, wholly combustible cartridge cases are extensively called for in modern ammunition. Nevertheless, in comparison with metal cartridge cases, these present a disadvantage where the leaktightness during firing at the rear of the shell is concerned. In fact, precisely because they are destroyed at the time of firing, combustible cartridge cases do not contribute any additional leaktightness to that offered by the breech closure device. In the usual weapon systems the closure device generally ensures effective leaktightness up to 3.times.10.sup.8 Pa, or 3000 bars. This limit is wholly acceptable in the case of most conventional shells and allows a wholly combustible cartridge case to be employed. However, this limit is insufficient for some shells such as the dart shells intended for piercing armour and which must be fired at higher pressures which can be up to 7.times.10.sup.8 Pa, or 7000 bars. In this case, if the breech of the weapon system is not to be modified and made heavier, the metal cartridge case offers the advantage of contributing the indispensable addition of leaktightness due to its metal base.
With this type of shell, a particularly advantageous solution is provided by semicombustible cartridge cases consisting, on the one hand, of a case-shaped combustible member and of a metal base. In this type of cartridge case the combustible case takes part in the combustion of the propellent charge, contributing additional energy and avoiding the retention of a large quantity of toxic gases, while the metal base ensures effective leaktightness at the rear of the projectile, even at very high pressures. There are essentially two types of semicombustible cartridge cases in existence. In the first type the combustible member is simply a combustible tube obtained advantageously by spiralling combustible paper, the joint with the metal base being made by means of an additional bottom part, as described, for example, in the addition 87,428 to French Patent 1,349,818. In the second type, described, for example, in French Patent 2,365,096, the combustible member comprises both a tubular part and a bottom which can fit directly into the metal base. Such a combustible member is preferably obtained by felting, by starting with an aqueous slurry containing the materials of which the combustible member is composed.
As can be seen from the abovementioned patents, the fastening of the combustible member to the metal base is effected by means of a fastening component, generally a fastening ring, which binds the bottom of the combustible member around a hollow internal collar carried by the metal base. This solution is practical, but necessarily means that the interior of the combustible member must be free at the time of the fastening of the metal base, to allow the introduction and the positioning of the said fastening component. Consequently, in currently known semicombustible cartridge cases the propellent powder can be introduced only after the metal base has been fitted. Furthermore, a correct ignition of large-calibre ammunition requires the presence of an ignition tube situated in the extension of the said hollow collar carried by the metal base and which, for obvious safety reasons, must preferably be fitted before the powder is introduced.
Under these conditions it is therefore not possible to ensure a correct filling with propellent powder of semicombustible ammunition through the bottom of the latter.
On the other hand, it is easy to fill semicombustible ammunition with powder through the open end of the tube of the combustible member, which is away from the metal base, and then to fit the shell. This solution is suitable in the case of conventional shells which have a rounded bottom which does not enter far into the combustible tube.
On the other hand, a new difficulty arises with dart shells which have finning entering deeply into the combustible tube. For obvious safety reasons these shells must be fastened to the combustible member before the powder is introduced. With this type of ammunition, since the introduction of the powder must be performed after the metal base and the shell have been positioned, this introduction can be performed only through a side opening made in the wall of the combustible tube, an opening which must then be closed again. Combustible materials do not lend themselves well to a cutout of this type, and the automation of the manufacture of dartshell semicombustible ammunition is at present practically impossible.